A stoichiometric air-fuel ratio with lambda equal to 1 in the exhaust gas is adjusted particularly with the aid of lambda sensors. In so doing, the lambda value indicates how far the actually prevailing air-fuel mixture deviates from the mass ratio of 14.7 kg air to 1 kg fuel, which is theoretically necessary for complete combustion. Lambda is in this case the quotient from the air mass, which is supplied, and the theoretical air requirement.
The mode of operation of the lambda probe is based on the principle of a galvanic oxygen concentration cell with a solid state electrolyte. The solid state electrolyte typically consists of two interfaces separated by a ceramic. The ceramic material used becomes conductive for oxygen ions at approximately 350° C., so that the so-called Nernst voltage is then produced when the percentage of oxygen is different on both sides of the ceramic, which is situated between the interfaces. This electrical voltage is a measurement of the ratio of the oxygen partial pressures on both sides of the ceramic. Because the residual oxygen content in the exhaust gas of an internal combustion engine is dependent to a great degree on the air-fuel ratio of the mixture supplied to the engine, it is possible to use the oxygen percentage in the exhaust gas as a measurement for the actually prevailing air-fuel ratio.
The operational temperature of the sensors is as a rule specified by the manufacturer and typically lies between 750° and 800° C.
Also in the case of other sensors, it is often necessary to heat the sensor up to an operational temperature when starting the engine. In order to receive utilizable measurement signals as early on as possible—preferably still during the warm-up phase of the internal combustion engine—it is desirable to heat up the sensor as quickly as possible.
When starting the internal combustion engine and particularly in this case a cold internal combustion engine, water vapor developing during combustion can condense on the cold surface areas of the exhaust gas tract in the form of water drops.
If a water drop hits the ceramic surface area of a sensor, the regional cooling down induced by the water drop can be so great that the ceramic is destroyed on account of the temperature differences and the thermal tensions connected with them.
A gas probe is known, for example, from the German patent DE 199 34 319 A1, which has a protective pipe to protect the ceramic sensor element. An additional inner pipe with openings for the admission and emission of the measurement gas, respectively the exhaust gas, is supposed to protect the ceramic sensor element from direct contact with the water.
A method for operating a gas probe is known from the patent 10 2004 035 230 A1, wherein operating states of the internal combustion engine are ascertained. When an operating state prevails, wherein a lower temperature is to be expected in the exhaust gas tract, i.e., for example, during a cold start-up, the sensor is adjusted to a lower temperature or completely switched off. This is done to counter the danger of a thermal shock caused by water. Consequently the sensor has no readiness for closed-loop control during start-up of the internal combustion engine.